Multiwell filtration devices have been disclosed in U.S. Pat. Nos. 4,777,021 and 4,427,415. Both reference devices have in common that liquid drops of samples penetrating separation layers will enter a common collecting vessel that is part of a vacuum chamber that is sealed by a support plate interconnecting the individual sample containers arranged in a matrix-like manner. In the reference device for separating liquids, the sample components retained by or in the separation layer are of interest in the subsequent tests. The liquid penetrating the separation layer is "lost" for further analyzing purposes. However, it is frequently necessary for a chemical or bio-polymer separation of samples to individually and selectively catch the sample components that have penetrated the separation layer or have been washed out or removed from the separation layer by applying a solvent. This cannot be realized by using the reference devices.
U.S. Pat. No. 4,902,481 discloses a multi-well filtration apparatus for the assay of microliter quantities. The apparatus is a filter which is positioned in each well on a plate having an open spout positioned in the plate. The spout has a collar on its outer surface extending in a direction perpendicular to the vertical axis of the spout. The collar prevents a liquid droplet from climbing the outer surface of the spout from its open end.
In the '481 patent, the individual collecting containers are only a small distance apart. Due to the distance of the collecting containers to the sample containers, there is a risk that parts of a fluid drop to be received by a collecting container, arranged below a sample container, will enter a neighboring collecting container, thereby contaminating the filtrate thereof. Further, the forming of the drops may not be uniform in the device of U.S. Pat. No. 4,902,481. In particular, it is not uniform when the vacuum in the chamber is released for a short time in order to replace the set of collecting containers accommodated therein by a new one. When releasing the partial vacuum in the chamber, the lower surface of the plate can become wetted by the liquid drops. When a partial vacuum is generated subsequently, relatively large drops will form since sucked-in liquid expands along the lower surface because the same is wetted. In this case, a drop may extend up to the annular collar, where it can be suctioned off via the gap between the annular collar and the collecting container. Thus, the liquid does not get into the collecting container, but may possibly run into an adjacent collecting container (contamination) or flows along the outside of the respective collecting container. Contamination of the liquid drops received by the collecting containers is particularly intolerable in the bio-polymer separation of liquid samples since, in this event, the examination of nucleic acids and proteins can be performed after a plurality (25 to 40) of self-replicating cycles, for example in a polymerase chain reaction (PCR), whereby even slight contaminations (contaminations of 1:1000) will be magnified and thus lead to erroneous results in the subsequent analysis.